The present invention relates to incandescent light bulbs, and more particularly, to constructions for such light bulbs designed to increase the amount of visible light that they emit or to increase the bulbs"" life-span.
A conventional incandescent light bulb, hereinafter sometimes referred to simply as a light bulb, comprises a coiled (usually helically wound) tungsten wire filament which is supported inside a glass globe or other suitable envelope. The ends of the wire filament are connected to, and supported by, the outer ends of corresponding current supply wires. The inner ends of the current supply wires are usually connected to terminals in a base of the light bulb to facilitate connection to a source of electrical power supplied to a socket or other connector in which the bulb is mounted. When current flows through the wire filament, it heats to a very high temperature and gives off visible light. A vacuum may be drawn on the glass envelope or it may be filled with an inert gas such as krypton, halogen or xenon which can contribute to the life-span and efficiency of the light bulb.
The power used by a light bulb is indicated by its rating in watts. This is a measure of how much power is consumed by the wire filament of the light bulb. The wattage rating of a light bulb is determined by several factors, including the thickness, length and configuration of its wire filament. The amperage at which electricity passes through the filament is directly proportional to an applied voltage. The power rating of a light bulb is usually expressed in terms of watts, with the wattage of a light bulb being the product of the bulb"" current rating in amps times the voltage being applied. By way of example, a light bulb might be specified with a rating of 6.00V/0.50 A, in which case the light bulb""power would be three Watts. At a given voltage, the higher the amperage, the greater the power, and theoretically, the greater the brightness of the light bulb.
In addition to the power supplied to a light bulb, its perceived brightness is affected by many other factors, including the construction of the light bulb (e.g. gas fill and pressure of fill), and the construction and shape of the filament. Perceived brightness is also a function of the color temperature of the light emitted by the filament.
There are scientific measurements of brightness called footcandles (or candela) and lumens. The brightness of a light bulb can be empirically determined in a laboratory in candela or lumens by carefully controlling the applied voltage, the mechanical factors and the environment around the light bulb. Such conditions are generally not reproducible in the field. Therefore, most light bulb manufacturers do not provide candela or lumen ratings for their light bulbs. The wattage rating for a light bulb therefore commonly provides a rough guide to its brightness.
Most incandescent light bulbs are designed to operate at a specific nominal or design voltage. If the voltage supplied is too high, the wire filament can melt and/or break. If the voltage is too low, the brightness of the light bulb will be diminished. A slight over-voltage, e.g. ten to fifteen percent, can make a light bulb glow much brighter, but at the expense of its life expectancy. A slight under-voltage will cause the light bulb to burn less brightly, but its life expectancy will be increased. A problem with light bulbs intended for use with battery power is that the voltage supplied is not stable. As an example, four standard Alkaline C cells may supply a combined six volts at the beginning of their life, but then the supplied voltage can eventually dip to under five volts as the batteries drain.
There are many applications where it would be desirable to provide an incandescent light bulb intended to be powered via batteries which will produce the maximum amount of brightness for a given power consumption. Such a light bulb must still provide a reasonable life expectancy, such as twenty to fifty hours of operating time. Foremost among these applications is the flashlight, whether used in the household, by law enforcement personnel and fireman, or in more exotic applications such as underwater diving.
It is therefore an object of the present invention to provide an improved incandescent light bulb that will provide increased illumination. It is another separate object of the present invention to provide an improved light bulb that provides a longer life expectancy for a given power consumption.
In accordance with the present invention, an incandescent light bulb includes a lamp envelope, a coiled filament disposed inside the lamp envelope, and current supply wires that electrically connect a pair of opposite terminal ends of the filament to a power supply. The power supply has a predetermined nominal voltage range for causing a current to flow through the filament sufficient to heat the same to a temperature at which it emits visible light. In one arrangement the filament has a pair of end segments and a central segment between the end segments. The end segments of the filament have a first pitch which is smaller than a second pitch of the central segment of the filament. With this configuration, more of the wire length of the filament will be heated to the highest operating temperature. Because the filament typically fails at the hottest point, the critical peak temperature of the filament which causes low voltage lamps to fail is eliminated and light bulb life expectancy is increased. The variable pitch filament increases the brightness of the light bulb without decreasing its life, or increases life without decreasing brightness.